Sounds like a dentist drill? I aint visiting while you are charging batteries. 
quote=wight
snip
I think that temperature delta termination is pretty rare but it’s legit
[/quote]
That was a good read, thx
Sorry auto quote was messed up…
Well, maybe a 12 volt would be good. I have always found that DC fans like that will operate in a range of voltage. Higher voltage may shorten their life span, no doubt. I will run it till it dies and then worry about it, but when I do replace it, I will use a 12 volt. I probably have one lying around, except it’s probably much bigger.
I like the idea, I might try to mod mine too. Can I install a potentiometer in series with the fan to be able to adjust the speed manually, depending on the number of cells I want to charge and so?
You could use a pot depending on wattage of fan (too high and it will burn the resistive tracks), but better to use PWM to control the speed, you will get more adjustability from a pot.
I found this which is only adding a 74AC14 IC, two caps and two diodes:
if I read the text correctly. O-L’s fan is 350ma so may fry the chip used in the link? That’s a neat circuit to build from so maybe add additional driver.
Thx
7805 5vdc regulator?
or the LM317 (which I see they have a SOT-223 1.5A SMD variant 
)
https://oshpark.com/shared_projects/IU9KGZF8
heck even one of those small DC-DC converters would work…but if you want crazy cheap…go with the 7805 or LM317
Yes you would need to bring down the supply voltage a bit for the IC.
of these, lol. I think we need a min of 7 volts, but yeah.
Just wrapping the capacitor leads around the two leads on the right then kapton taping it to the chip would make it much smaller, then just a blob of hot glue to the lip under the top and bottom of the fan and viola…tiny voltage regulator to run the fan at a “alot quieter” speed, still getting enough airflow to keep the internals cooler but not loud enough to be a dental drill
Get a cheapo 40mm fan, and a regulator like that above and viola
http://www.directron.com/fnsx0442.html
http://www.directron.com/ec4010l12c.html
Found another very inexpensive 40x40mm fan
http://www.ebay.com/itm/New-DC-12V-2Pin-Mini-Small-Cooling-Fan-40mm-40mm-10mm-For-Computer-3D-Printer-/141218895281?pt=US_Computer_Case_Fans&hash=item20e14d55b1
Ick. IMO fans are something not to skimp on, similar to batteries. If I use a fan to cool something I want it to be fairly dependable. (If noise level/signature bothers you then not skimping becomes even more important)
Even for a mod fan?
My personal preference is for [my own] things to be fanless up until the point where a fan is necessary. If a fan is ‘necessary’ I sure don’t want it to stop spinning.